I can't select a bone on weight-paint
An orange outline indicates object mode selection for the armature. You want the armature in pose mode (blue select outline) before you start weight painting so that you can select and move the bones while you paint.


Select the armature and go into pose mode.
Select the mesh and go into weight paint mode.


When weight painting, the action mouse button setting makes a difference.


 LMB will weight paint


If you are using Select with Right -


 RMB will select a bone


If you are using Select with Left -


&#9096; Ctrl LMB will select a bone


While weight painting and the armature is in pose mode, G/S/R works with the active bone, not the object being painted.